In modern commercial aircraft, an air cycle environmental control system is provided to suitably condition air to be supplied to the cabin or cockpit or other locations for occupant comfort. Typically, such air cycle systems condition a flow of pressurized air, for example bleed air from the aircraft engine, by not only regulating the pressure of the air to a desired level for cabin pressurization, but also by cooling and dehumidifying the air. Such systems are disclosed, for example, in commonly assigned U.S. Pat. Nos. 4,209,993 and 4,374,469, both of Rannenberg, and U.S. Pat. No. 4,430,867 of Warner.
As disclosed in these patents, the flow of compressed air to be conditioned is passed through the compressor section of an air cycle machine, thence further cooled to cause condensation of moisture in the air, thereby dehumidifying the air, prior to being expanded through the turbine section of the air cycle machine to a desired pressure level for delivery to its point of use, e.g. the aircraft passenger or pilot cabin. Typically, the air discharged from the turbine section of the air cycle machine is reheated prior to its delivery to its point of use by passing the turbine exhaust air in indirect heat exchange relationship with the compressed air being directed to the turbine thereby further cooling the compressed air and heating the turbine exhaust air.
In the air cycle machine, the turbine and the compressor thereof are mounted to a common shaft which is driven by the expansion of the compressed air through the turbine. Generally, an axial flow fan is also mounted to this common drive shaft at one end thereof and is therefore, like the compressor, driven by the turbine. This fan is typically utilized to draw a flow of cooling air through an upstream heat exchanger wherein the compressed air being conditioned is cooled by passing in indirect heat exchange relationship with the cooling air. Most commonly, the cooling air, typically ram air, enters the heat exchanger from outside the aircraft, passes through the heat exchanger and enters a fan inlet and diffuser housing which provides both an inlet flow passage for directing the cooling air through the fan and a diverging outlet flow passageway through which the exhaust air discharging from the fan is vented overboard.
The maximum load on such air cycle systems is experienced when the aircraft is on the ground on a hot day. Under such conditions, the fan operates under its highest load to draw a maximum flow of ambient air through the heat exchanger as cooling air and to vent the air back overboard. However, when in flight at normal cruise altitudes, the ambient air is much cooler, cabin cooling requirements are substantially reduced, and the ram pressure provides a substantial portion of the energy necessary to drive the ambient cooling air through the heat exchanger. Thus, the air cycle machine fan need only operate at a low power level to induce adequate cooling air flow through the heat exchanger. At altitudes between ground level and cruise, the fan will operate at intermediate load ratings selected to induce adequate cooling air flow through the heat exchanger so as to conserve power consumption which in turn results in lower fuel expenditure.
In order to provide for varying amounts of flow through the fan over the operating load range, it is desirable, and customary, in such prior art air cycle cooling systems to provide a fan bypass duct which directs a portion of the cooling air having passed through the heat exchanger around the fan rather the passing all of the cooling air flow through the fan. In order to control the amount of flow passing through the bypass, which varies from little or no bypass flow at ground to a substantial percentage of the total cooling air flow at cruise altitude, a bypass check valve must be provided in operative association with the bypass duct. Of course, the need for a fan bypass duct with a bypass check valve adds to the complexity of the air cycle cooling system as well as increasing its size and weight, all undesirable complications in aircraft applications where simplicity, low volume and low weight are desired.